The present invention relates to a power circuit that allows a battery to supply voltage stably to a load in a vehicle with an idling stop control.
Recently a vehicle with an idling stop control has been put to practical use for reducing fuel consumption and exhaust gas emission. The idling stop control of the vehicle turns off its engine automatically when it is detected that the vehicle stops e.g. at a traffic light and restarts the engine automatically when it is detected that the vehicle is about to start again.
When the engine of such vehicle is restarted after an idling stop, a large current flows to a starter motor of the vehicle, so that the voltage of a battery of the vehicle is dropped temporarily. Accordingly, the voltage of power to various loads such as electronic devices other than the starter motor which are connected to the battery is also dropped temporarily at a restart of the vehicle. Therefore, there is a fear that the voltage falls below a level that is required for any electronic device to operate properly, thus inviting a temporary malfunction of the device. For example, a car navigation system and an audio system may be reset or a sound may jump in the audio system. Thus, unforeseeable operation of any electronic device may occur.
To solve the above problem, a power circuit may be provided between the battery and the load so as to keep constant the voltage required for the loads even when a voltage drop of the battery occurs temporarily.
Japanese Patent Application Publication 2005-112250 discloses a power circuit to solve the above problem. The power circuit includes a boost circuit and a bypass relay that bypasses the boost circuit when the relay is closed. At a restart of an engine after an idling stop, the boost circuit is activated with the bypass relay kept opened, so that the voltage of a battery is boosted by the boost circuit and the boosted voltage is supplied to various electronic loads of the vehicle. During a normal state other than the restart of the engine, the bypass relay is kept closed thereby to bypass the boost circuit, so that the voltage of the battery is supplied to the loads through the bypass relay.
When the voltage of the battery is dropped temporarily at the restart of the engine after an idling stop, the power circuit ensures that the voltage required for the loads is supplied. Meanwhile, during the normal state other than the restart of the engine, the voltage of the battery is supplied to the loads without being dropped by a switching element in the boost circuit, so that supply of the voltage required for the loads can be maintained.
However, if an open fault occurs in the bypass relay in the power circuit and the voltage of the battery is supplied at all times to the loads through the boost circuit, the voltage of the battery to be supplied to the loads is dropped by the switching element in the boost circuit.
The present invention is directed to providing a power circuit that allows supply of a stable voltage to loads of electronic devices of a vehicle equipped with an idling stop control.